Move Fluid template pre-processor and expression node types registrations to TYPO3_CONF_VARS
Currently these implementations are hardcoded into the TYPO3 CMS rendering context, which means the only ways to change these is to override the RenderingContext or use the public API of RenderingContext (which isn't always possible to do in TYPO3, e.g. FLUIDTEMPLATE doesn't really have any initialization methods you could use).
Moving these registrations to TYPO3_CONF_VARS allows extensions to add new such processor/expression types globally.
This new global registration will be vital to things like the frontend_editing extension to enable automatic editing of, for example, Extbase domain objects in any template instead of requiring template overrides which enable editing (such as it currently does).
Updated by Stefan Neufeind about 3 years ago
Some details from Claus, as he wrote in the 8.7-review on gerrit:
As mentioned elsewhere, this is a pre-requisite for improving the Fluid integration in the frontend_editing extension in several ways, without requiring each extension to opt-in to such capabilities. See https://gist.github.com/NamelessCoder/9aca683920d9e4225f0527e0ce900ba8
It is highly critical at least for this one extension - and definitely useful for others as well, who do such overrides but prefer to keep them in static configuration.